RFC: the registry follows the pins (CI check + regeneration in the bump workflows)

[bong-water-water-bong]

Problem

registry/architectures.json records the pins it was built from, and the site, the census and the
weekly release read it. But nothing regenerates it when a pin moves. Today two bumps skipped it:
#132 (ZINC, by the bump workflow) and #161 (llama.cpp-vulkan, by hand). The registry then named
stale pins, and docs/registry.md had drifted to counts from before three pin moves (265 instead of
323 architectures). Both had to be fixed by hand (#151, #162).

Design

• tools/registry_build.py --check-pins: compares the pins the registry records with the committed
  gitlinks. It needs no checkouts and no network. It runs as ctest registry_pins, next to
  registry_gaps, so CI fails a PR whose pin moved without the registry.
• scripts/registry-regen.sh: fetches the three trees the registry reads (llama.cpp,
  llama.cpp-vulkan, zinc) at their pins, depth 1. It regenerates the registry, amends HEAD, and
  re-runs the check.
• bump-zinc.yml, bump-llama-vulkan.yml and bump-hrx.yml run it right after committing the new
  pin, so automated bump PRs pass.
• docs/registry.md gives a dated snapshot and points to the JSON for current counts, instead of
  naming pins that go stale.

Alternatives

• Rebuild the whole registry in CI (--check): needs three large checkouts on every PR. The
  pin comparison catches the same drift in milliseconds.
• Regenerate on main after merge: then main is wrong between merges, and a bot pushes to
  main.

How it will be checked

• On main: passes.
• At 8a82fce1c (Bump ZINC: zolotukhin/zinc 29bc350ac4cb #132): fails with zinc recorded at 3a35e76d6, pinned at 29bc350ac.
• scripts/registry-regen.sh run on a worktree at that commit amends it with the regenerated
  registry (zinc 29bc350ac, counts unchanged), and the check then passes.

Dependencies, licences, security

• No new dependency.
• The workflows' permissions are unchanged. The script only fetches public repos that the
  workflows already clone.

Open questions

None.

[bong-water-water-bong]

Accepted by the maintainer on 2026-09-26. Implemented in #165.